Seattle 'intraslab' quake began deep in Earth

A rumble from far below

By Ann Kellan
CNN Science Correspondent

(CNN) -- The tremor that shook the Seattle and Olympia areas in Washington state Wednesday is the same kind of earthquake that killed dozens in Mexico in 1999 and claimed 1,000 lives in El Salvador this past January.

These poorly understood tremors, called "intraslab" quakes, pose a special threat to the Seattle area.

"The thing that's of interest here is that the larger of the intraslab earthquakes occur right beneath the urban corridor," said Steve Kirby from the United States Geological Survey. "So they're right beneath the feet of where people live."

MESSAGE BOARD Seattle shake-up   MAGNITUDE GUIDE Below is an interactive guide to earthquake magnitude and severity: Magnitude: Less than 3.5 3.5 - 5.4 Under 6.0 6.1 - 6.9 7.0 - 7.9 8 or greater

Intraslab quakes begin deep in the earth, in undersea areas called subduction zones.

In a subduction zone, one slab of the earth's surface is slowly sliding under another plate. As it sinks, it is under enormous pressure, and is heated by intense temperatures deep inside the earth.

That causes chemical and physical changes in the slab, which release trapped water. The water lets the rock crack along ancient fault lines, setting off an earthquake inside the slab anywhere from 30 to 180 miles below Earth's surface.

The quakes do less damage at that depth than they would at the surface, but that hardly means they are not powerful or deadly. Quakes of this type in Washington state in 1965 and 1949 each killed eight people.

Because they originate so deep below the Earth's surface, intraslab quakes are harder to study. Scientists believe this latest one in Seattle occurred about 30 miles underground.

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